Method for manufacturing silicon nitride sintered body

ABSTRACT

The present invention provides a silicon nitride sintered body having characteristics such as excellent wear resistance, a method for manufacturing the sintered body, and a cutting insert formed thereof. The silicon nitride sintered body is formed of a polycrystalline sintered body of predominantly β-Si 3  N 4 , wherein the oxygen content is 1.2-1.5 wt. %. The method for manufacturing the silicon nitride sintered body includes the following steps: adding an organic binder to a composition containing silicon nitride as an essential component and exhibiting a theoretical oxygen content of 2.0-3.0 wt. %; heating to remove the binder; introducing an oxygen-containing gas so as to control the carbon content to 0.10-0.60 wt. %; and sintering a resultant compact in an nitrogen atmosphere to control the oxygen content to 1.2-1.5 wt. %. The cutting insert of the present invention has excellent wear resistance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silicon nitride sintered body usedfor wear-resistant parts such as cutting inserts, bearings, and balls; amethod for manufacturing the silicon nitride sintered body; and acutting insert formed of the silicon nitride sintered body.

2. Background Art

Conventionally, silicon nitride sintered bodies predominantly containingsilicon nitride and having an excellent strength have been used forcutting inserts, wear-resistant parts, etc.

In recent years, for the case where silicon nitride is used for cuttinginserts, there has been proposed a technique of decreasing the amount ofa sintering aid (an oxide) which is added to the essential component, inorder to enhance wear resistance of the sintered body (See PCT KohyoPublication 8-503664 or U.S. Pat. No. 5,525,134). As disclosed therein,a sintered body contains at least about 85 volume percent (vol. %)silicon nitride and less than 5 vol. % intergranular phase. The ceramichas greater than 0.2 weight percent (wgt. %) magnesia, greater than 0.2wgt. % yttria wherein the sum of magnesia and yttria is less than 5 wgt.%. The ceramic also includes between 1.3 wgt. % to 2.2 wgt. % oxygen.

However, with the conventional cutting insert, there is a drawback thatan extreme decrease in the amount of the sintering aid to be addedrenders sinterability lower than expected and decreases wear resistanceand chipping resistance of the sintered body.

SUMMARY OF THE INVENTION

The present invention was made in view of the foregoing, and an objectof the present invention is to provide a silicon nitride sintered bodyhaving properties such as excellent wear resistance and a method formanufacturing the same. The present invention also provides a cuttinginsert formed of the sintered body.

According to a first aspect of the present invention, there is provideda polycrystalline sintered body comprising predominantly β-Si₃ N₄, i.e.between about 95 to about 97% by volume, wherein the oxygen content is1.2-1.5 wt. %.

The silicon nitride sintered body of the first aspect of this inventionhas a dense texture, high hardness, and excellent wear resistance andchipping resistance.

According to a second aspect of the present invention, there is provideda method for manufacturing the above-described silicon nitride sinteredbody including the following steps: adding an organic binder to acomposition containing silicon nitride as an essential component andexhibiting a theoretical oxygen content of 2.0-2.3 wt. %; forming ashaped or pressed body therefrom; heating to remove the binder (e.g.,forming a calcined body); introducing an oxygen-containing gas so as tocontrol the carbon content to 0.10-0.60 wt. % (e.g., cooling thecalcined body); and sintering the resultant compact in a nitrogenatmosphere to control the oxygen content to 1.2-1.5 wt. % (e.g., primaryand secondary sintering steps).

In the method of manufacturing the sintered body of this invention, whenprimary sintering is performed under pressurization, decomposition ofsilicon nitride is prevented. Thus, high-temperature sintering isrealized. Moreover, pores are effectively closed by the high-temperaturesintering at, for example, subsequent secondary sintering through HIP.

According to a third aspect of the present invention, there is provideda cutting insert formed of the silicon nitride sintered body of thefirst aspect.

The cutting insert of the third aspect of this invention has a densetexture, high hardness, and excellent wear resistance and chippingresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomebetter understood by reference to the following detailed description ofthe preferred embodiment in connection with the accompanying drawings,in which:

FIG. 1 is an explanatory view showing the change of texture of thesintered body during a firing step, etc.; and

FIG. 2 is an explanatory view showing different types of wear.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT THEREOF

The polycrystalline sintered body of the present invention ispredominantly β-Si₃ N₄, wherein the oxygen content is between 1.2-1.5wt. %.

As clarified in the following Examples, the silicon nitride sinteredbody of the present invention has a dense texture, high hardness value,and excellent wear resistance and chipping resistance, since thesintered body has an appropriately controlled oxygen content of 1.2-1.5wt. %. The sintered body is also characterized as having minipores of 10microns or less observable in an amount of 0.02% by volume or less.

The method for manufacturing the above-described silicon nitridesintered body according to the present invention comprises the followingsteps: adding an organic binder to a composition containing siliconnitride as an essential component and exhibiting a theoretical oxygencontent of 2.0-2.3 wt. %; forming a shaped or pressed body therefrom;heating to remove the binder (e.g., forming a calcined body);introducing an oxygen-containing gas so as to control the carbon contentto 0. 10-0.60 wt. % (e.g., cooling the calcined body); and sintering theresultant compact in a nitrogen atmosphere to control the oxygen contentto 1.2-1.5 wt. % (e.g., primary and secondary sintering steps).

In the present invention, a sintering aid is added in a minimum amountthat enables sintering so that the texture is densified and a grainboundary phase (glass phase) is reacted with carbon remaining in thecompact, to thereby decrease the amount of the grain boundary phase.

Specifically, as schematically shown in FIG. 1, a calcined bodyincorporated with a predetermined amount of carbon undergoes primary andsecondary sintering to remove carbon in the forms of carbon dioxide andcarbon monooxide by combining with oxygen in order to perform densesintering. Thus, a silicon nitride sintered body having high density andexcellent wear resistance can be obtained, since decrease of a grainboundary phase and densification are simultaneously attained in theabove-described process.

Reasons for limiting the above-described values will next be described.

(1) Theoretical oxygen content; 2.0-2.3 wt. %

The theoretical oxygen content relates to an amount of a sinteringagent. When the content is less than 2.0 wt. %, densification isdifficult, whereas when it is in excess of 2.3 wt. %, a grain boundarypredominates to lower the wear resistance of the sintered body.

(2) Carbon content; 0.10-0.60 wt. %

When the carbon content in the calcined body is less than 0.10 wt. %,reaction to decrease a grain boundary phase is insignificant and theamount of a grain boundary phase therefore becomes considerably high tolower the wear resistance of the sintered body, whereas when it is inexcess of 0.60 wt. %, the wear resistance of the sintered bodydecreases.

(3) Oxygen content; 1.2-1.5 wt. %

When the oxygen content is less than 1.2 wt. %, a grain boundary isformed in an excessively low amount to result in poor densification,whereas when it is in excess of 1.5 wt. %, the wear resistance of thesintered body decreases.

In the method for manufacturing the silicon nitride sintered body of thepresent invention, sintering in a nitrogen atmosphere is preferablyperformed under pressurization.

The present invention exemplifies conditions for sintering in a nitrogenatmosphere. For example, primary sintering performed underpressurization of several atm prevents silicon nitride from decomposingto enable sintering at high temperature. Moreover, pores are effectivelyclosed by the high-temperature sintering in subsequent secondarysintering performed through, for example, HIP (hot isostatic press).

In the method for manufacturing the silicon nitride sintered body of thepresent invention, preferably, a microcrystalline-wax-type organicbinder is used as an organic binder.

The present invention exemplifies species of the organic binder, and amicrocrystalline-wax-type organic binder may be used as the organicbinder. The use of the organic binder advantageously enhancesflowability of powders and strength of shaped or pressed bodies.

In the method for manufacturing the silicon nitride sintered body of thepresent invention, an oxygen-containing gas is preferably introduced ina cooling step performed after removal of the organic binder by heating.

The present invention exemplifies timing to introduce theoxygen-containing gas, which is performed in a cooling step performedafter removal of the organic binder by heating. This method enablessimultaneous introduction of the oxygen-containing gas and cooling aftercalcination and facilitates operation steps. Also, carbon is effectivelyincorporated into a calcined body by introducing the oxygen-containinggas in the cooling step. Furthermore, the amount of carbon to beincorporated may be easily regulated by regulating conditions (e.g.,flow rate) of the oxygen-containing gas to be introduced.

In the method for manufacturing the silicon nitride sintered body of thepresent invention, air is used as the oxygen-containing gas.

The present invention exemplifies species of the oxygen-containinggases, and use of air facilitates the manufacturing steps.

The cutting insert of the present invention has a dense texture, highhardness value, and excellent wear resistance and chipping resistance,since the sintered body that constitutes the cutting insert has anappropriately controlled oxygen content of 1.2-1.5 wt. %.

Embodiments of the invention, i.e., the silicon nitride sintered body ofthe present invention, the method for manufacturing the sintered body,and inserts made of the silicon nitride sintered body, will bedescribed.

a) First is described the method for manufacturing the silicon nitridesintered body in the present Example.

α-Si₃ N₄ having an average grain size of 1.0 μm or less (oxygen content:1.3 wt. %) as an essential component and MgO, Al₂ O₃, and Yb₂ O₃, eachhaving an average grain size of 1.0 μm or less and serving as asintering aid, were weighed at ratios specified in Table 1 shown below.The ratio of raw materials was determined such that the theoreticaloxygen content in the weighed composition was regulated to 2.0-2.3 wt.%.

The thus-weighed material was mixed with ethanol as a solvent for 96hours by use of an Si₃ N₄ ball and a pot having an inner wall made ofSi₃ N₄, so as to form a slurry.

The slurry was passed through a 325-mesh-sieve, and amicrocrystalline-wax-type organic binder dissolved in ethanol was addedin an amount of 5.0 wt. % to the sieved slurry. The resultant mixturewas spray-dried.

The obtained granulated powder was press-shaped to a shape identified asSNGN120408 (ISO standard), then the compact was placed in a heatingapparatus and calcined. The calcination was performed at 600° C. for 60minutes in a nitrogen atmosphere to dewax.

Moreover, in a cooling step during calcination air was introduced intothe heating apparatus in an amount of 3-60 L/min as specified in Table1, such that the carbon content in the dewaxed compact was regulated to0.10-0.60 wt. %.

Primary sintering was then performed at 1800-1900° C. for 240 minutes ina nitrogen atmosphere having a predetermined pressure of 3 atm.

Subsequently, secondary sintering was performed through HIP to finallyobtain the silicon nitride sintered body. The secondary sintering wasperformed at 1600-1700° C. for 120 minutes in a nitrogen atmospherehaving a predetermined pressure of 1000 atm.

The oxygen content of the sintered body was regulated to 1.2-1.5 wt. %through combination of primary and secondary sintering steps.

An X-ray diffraction analysis confirmed that α-Si₃ N₄ used as a rawmaterial had been converted to β-Si₃ N₄ through the sintering steps.

The thus-obtained silicon nitride sintered bodies of the presentExamples are polycrystalline, comprise predominantly β-Si₃ N₄ in anamount of 95-97 vol. %, and have an oxygen content of 1.2-1.5 wt. %.

Therefore, the sintered bodies have excellent characteristics such as adense texture, high hardness, high fracture toughness, and high wearresistance.

b) Next, there are described Test Examples to confirm effects of thesilicon nitride sintered bodies in the present Examples.

Each of the silicon nitride sintered bodies obtained through theabove-described method for manufacturing was processed by polishing toform cutting inserts having a shape identified as SNGN120408 (ISOstandard). These cutting inserts were used as sample Nos. 1 to 5.

As Comparative Examples, silicon nitride sintered bodies weremanufactured and similarly processed in accordance with compositions andconditions specified in Table 1, to thereby serve as sample Nos. 6 and7, each of which has a carbon content (residual carbon) and an oxygencontent outside the scope of the present invention.

The below-described measurements and evaluations of (1) to (4) wereconducted on the above-described sample Nos. 1 to 7.

(1) <Measurement of the carbon content (residual carbon content) aftercalcination>

Each of the calcined bodies was crushed to a particle size of φ 1 [mm]or less. The crushed product was weighed through burning in an oxygenstream-infrared absorption method in accordance with JIS-Z2615.

(2) <Measurement of the oxygen content in the sintered bodies>

Each of the sintered bodies was crushed to a particle size of φ 1 [mm]or less and fused by heating in an inert gas. The determination wasperformed through a non-dispersive infrared absorption method.

(3) <Physical properties>

(Apparent porosity)

A cross-section of each sample was mirror-polished and the surface wasexamined by use of a metallurgical microscope to thereby classifyapparent porosity in accordance with CIS-006B-1983, which is registeredas "Classification Standards for Apparent Porosity of Cemented Carbides"and corresponds to ISO 4505.

(Vickers hardness Hv)

Measurement conditions: depressor-indentation pressure; 30 kg andindentation time; 15 sec.

(Fracture toughness Kc)

Measured through an IF method specified in JIS RI 607.

(4) <Evaluation of cutting performance>

In accordance with the below-described conditions, an edge face of acylindrical cast iron workpiece was continuously machined under dryconditions and the maximum wear depth (VB_(max)) in flank wear depths(VB) (see FIG. 2) was measured at the tip of a cutting tool after 20minutes' processing.

Workpiece material; JIS FC200

Workpiece shape; outer diameter φ 240 mm×inner diameter φ 180 mm

Cutting speed; V=300 m/min

Feed per revolution; f=0.34 mm/rev

Depth of cut; d=0.2 mm

Measurement results of the above-described (1) to (4) are shown in Table2 together with calcination conditions (heating conditions for removingan organic binder), cooling conditions (flow rate of air feeding forcooling the calcined body), conditions of primary sintering (temperatureconditions of primary sintering), and conditions of secondary sintering(heating conditions at HIP), as shown in Table 3.

                  TABLE 1                                                         ______________________________________                                               Feed composition [wt. %]                                                                 Essential                                                          Sintering aid                                                                              component Theor. oxygen                                          MgO  Al.sub.2 O.sub.3                                                                      Yb.sub.2 O.sub.3                                                                      Si.sub.3 N.sub.4                                                                      content [wt.%]                            ______________________________________                                        Example                                                                              1     0.3    1     3     95.7    2.19                                         2     0.3    1     3     95.7    2.19                                         3     0.3    1     3     95.7    2.19                                         4     0.5    1     1     97.5    2.06                                         5     1      1     1     97.0    2.25                                  Comp. Ex.                                                                            6     0.3    1     3     95.7    2.19                                         7     0.3    1     3     95.7    2.19                                  ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                       Cooling                                                               Heating condition Sintering conditions                                        condition                                                                             at calcination                                                                          Primary  Secondary                                          at calcination                                                                        Air flow rate                                                                           sintering                                                                              sintering (HIP)                                    [° C.]                                                                         [L/min]   [° C.]                                                                          [° C.]                               ______________________________________                                        Example                                                                              1     600        3      1850   1600                                           2     600       30      1850   1600                                           3     600       60      1850   1600                                           4     600       30      1850   1600                                           5     600       30      1900   1700                                    Comp.  6     600        0      1900   1700                                    Example                                                                              7     600       100     1850   1600                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                                            (4)                                                    (2)                    Cutting                                   (1)          Oxygen  (3)            perform-                                  Residual     content              Fracture                                                                            ance                                  carbon       of              Hard-                                                                              tough-                                                                              VBmax                                 content      sintered                                                                              Micro-  ness ness  wear                                  [wt. %]      body    pore*   Hv   Kc    [mm]                                  ______________________________________                                        Ex.   1     0.42     1.20  A02   1550 7.5   0.31                                    2     0.25     1.37  A02   1580 7.3   0.43                                    3     0.11     1.48  A02   1600 7.0   0.52                                    4     0.22     1.22  A02   1650 6.7   0.36                                    5     0.36     1.25  A02   1640 6.8   0.36                              Comp. 6     0.68     1.16  A08   1340 --    chipping                          Ex.   7     0.09     1.54  A02   1430 7.9   0.94                              ______________________________________                                         *A02 refers to the case in which pinholes having a size of 10 μm or        less are observed in amounts of 0.02% by volume.                              A08 refers to the case in which pinholes having a size of 10 μm or les     are observed in amounts of 0.6% by volume.                               

As is apparent from Tables 1 to 3, sample Nos. 1 to 5, which fall withinthe scope of the present invention, are manufactured by the followingsteps: adding a sintering aid to silicon nitride as an essentialcomponent to thereby prepare a composition having a theoretical oxygencontent of 2.0-2.3 wt. %; adding a microcrystalline-wax-type organicbinder thereto to form a compact; heating the compact to remove theorganic binder (calcination); cooling by introducing air so as toregulate the carbon content to about 0.10 to about 0.60 wt. %; sinteringunder pressurization in a nitrogen atmosphere (primary sintering) toregulate the oxygen content of the sintered body to 1.2-1.5 wt. %; andsecondarily sintering through HIP.

Thus, the silicon nitride sintered bodies of sample Nos. 1 to 5 have anapparent porosity (micropore) of A02 (classification standards, CIS006B-1983) and sufficient density. The hardness Hv is as high as 1550 ormore and the fracture toughness Kc is as high as 6.7 or more.Furthermore, with regard to the cutting performance thereof, thesintered bodies have a VBmax wear depth of as low as 0.52 mm or less andsufficient wear resistance.

In contrast, sample No. 6 serving as a Comparative Example, which wascalcined without air flow, has a residual carbon content as high as 0.68wt. %. Therefore, the oxygen content in the silicon nitride sinteredbody is as low as 1.16 wt. %, which results in the apparent porosity(micropore) of A08 (classification standards, CIS 006B-1983) and poordensity. The hardness Hv is as low as 1340. Moreover, disadvantageouschipping occurs during cutting.

Sample No. 7 serving as a Comparative Example, which was calcined underair flow of as much as 100 L/min, has a residual carbon content as lowas 0.09 wt. %. Therefore, the oxygen content in the silicon nitridesintered body is as high as 1.54 wt. %, which results in a hardness Hvas low as 1430. Furthermore, with regard to the cutting performancethereof, the sintered body has a VBmax wear depth of as much as 0.94 mmor less and poor wear resistance.

FIG. 1 is a schematic illustration which illustrates the change intexture of a sintered body during the firing step. As illustratedtherein, a calcined body of silicon nitride, a sintering aid and carbonare combined (see 2). During sintering (4), the generation of a grainboundary phase progresses to densification (6). During sintering, carbonand oxygen are driven off as carbon dioxide and carbon monoxide untildensification is complete.

As illustrated in FIG. 2, a ceramic metal cutting insert is composed ofsilicon nitride ceramic material as disclosed herein. The insert 10 isparticularly useful in roughing and interrupted cutting of metals wherea combination of high toughness and high wear resistance is required.

The insert 10 includes a rake face 3 over which chips formed duringmachining flow. The insert 10 also includes flank faces 5 and cuttingedges 7 which are formed at the junction of the rake face 3 and flankfaces 5. As illustrated the rake face 3 includes face or crater wear 22and crater wear amount KT. FIG. 2 also illustrates flank wear 18 andwear amount VB. The figure also illustrates nose (triangular wear 14,flank wear 16, 18 and end cutting edge wear 12.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the claims, the presentinvention may be practiced other than as specifically described herein.

The present disclosure relates to subject matter contained in JapanesePatent Application No. HEI 9-360547, filed on Dec. 26, 1997, which isexpressly incorporated herein by reference in its entirety.

What is claimed is:
 1. A method for manufacturing a polycrystalllinesintered body of between about 95-97 percent by volume β-Si₃ N₄ with anoxygen content of about 1.2 to 1.5 percent by weight, said methodcomprising the steps of:adding an organic binder to a compositioncontaining silicon nitride as an essential component and exhibiting atheoretical oxygen content of 2.0-2.3 wt. %; heating to remove theorganic binder; introducing an oxygen-containing gas with an air flowrate of 3 to 60 L/min in a cooling step after removal of the organicbinder so as to control the carbon content to 0.1-0.6 wt. %; andsintering the resultant compact in a nitrogen atmosphere to control theoxygen content to 1.2-1.5 wt. %.
 2. The method according to claim 1,wherein the sintering in a nitrogen atmosphere is performed underpressurization.
 3. The method according to claim 1, wherein amicrocrystalline-wax organic binder is used as the organic binder. 4.The method according to claim 1, wherein air is used as theoxygen-containing gas.